The vision of realizing Internet of Things (IoT) pervasively connecting large number of sensors and devices requires development of novel solutions that scavenges ambient energy to supply the power required for the operation of the sensors. Relying on batteries as the source of energy for wireless sensors impose several limitations including the need for routine maintenance/charging of batteries, operation interruption and cost involved in replacing batteries specially those employed in harsh environments, and challenges of scaling of battery-powered wireless sensors to millions of nodes. RF energy from ambient sources can be used to power efficiently the sensor networks with or without batteries. In addition, they can also be used to partially/fully charge other portable electronic devices to extend their battery life.
There is accordingly a growing interest in harvesting ambient energy to partially/fully supply the energy required for the operation of portable electronic devices. Scavenging energy from the ambient electromagnetic wave referred as RF energy harvesting is one of the most popular method for powering low-power wireless sensors. As most of today's integrated circuits are fabricated in CMOS technology, it is highly desirable to integrate RF energy harvesting systems with the rest of the system on a single CMOS chip for reduced cost and small form factor. RF powered devices are widely used in wireless sensor networks operating at power levels of 10 to 100's of μW. RF energy harvesting can be used in applications where the traditional energy sources such as light, vibrations are not available.
In an RF energy harvesting system 20 as shown in FIG. 1, an antenna 22 receives the incident RF signal, an impedance matching circuit 24 maximizes the power transfer from the antenna to the power converter 26, and a RF-DC power conversion circuit 26 converts the incident RF power to DC output power. The output DC voltage is stored in an energy storage component 28 (battery or capacitor) or can directly power the wireless sensors. The major challenge of scavenging RF energy is the limited signal strength of the RF waves and the low efficiency of the harvesting circuit at low input power.
The radiated signals are received by the RF energy harvesting system 20 and converted into a DC output voltage which is used to power-up the system. The radiated signals have to adhere to the regulation standards set by the federal communication commission (FCC). The unlicensed industrial, scientific and medical (ISM) band with a center frequency of 915 MHz operating at 902-928 MHz with a maximum effective isotropic radiated power (EIRP) of 4 W can be used for RF energy harvesting. The major challenge of scavenging RF energy is the limited signal strength of the RF waves and the low efficiency of the harvesting circuit. The signal strength is limited due to the attenuation in the signal due to the free space loss. Also, the power at the transmitter level cannot be increased without violating the FCC guidelines. Hence modification has to be done at the receiver side. The power consumption for the commercial sensor network nodes vary based on the manufacturers and has been estimated by various authors to be around 10 to 100 μW depending on the sensing application and the radio protocol. A multi-volt supply voltage is typically required for the operation of the sensor circuitry.
The power harvester unit 20 comprising a multi-stage rectifier 26 is a key component in RF energy harvesting systems. It converts the incoming weak RF signal into a DC voltage. The performance of the rectifier unit 26 can be evaluated based on its power conversion efficiency (PCE) which is the ratio of power delivered to the load to the input power, based on sensitivity i.e. the minimum input power required for production of a DC voltage at the output and finally based on output DC voltage levels. To increase rectifier's PCE, the energy losses such as those introduced by the non-zero ON resistances of the rectifying devices must be reduced. To increase sensitivity and output voltage levels, rectifying devices with lower threshold voltages are required. Hence, these performance parameters of the power harvester are strongly affected by the threshold voltage of the rectifying devices. The power level has to exceed the threshold voltage of the rectifying device to turn on the rectifier unit in the RF-DC power conversion circuit. The minimum power is referred as the power-up threshold of the system. In terms of voltage level, a minimum input voltage is required for the circuit operation and is referred as the dead-zone of the rectifier.
The performance of an RF energy harvesting system 20 is significantly affected by the threshold voltage of the rectifying device 26, the voltage that is required to turn on the semiconductor devices used as rectifying devices. A rectifying device 26 with lower threshold voltage enables the operation of RF-DC power converter 26 at low input power levels significantly reducing the power-up threshold of the rectifiers, and increasing the output voltage level for the same input power. The threshold voltage of device can be reduced using different technology-based approaches for the devices including silicon-on-sapphire (SOS), schottky diodes such as silicon-titanium schottky diodes or SMS and the HSMS diodes, special low-threshold-voltage transistors in CMOS process, and floating gate transistors which store a pre-charged voltage at the gate to lower the threshold voltage. The drawback of using technology-based approaches is additional fabrication steps that increases the production cost and prevents integration of RF energy harvester in mainstream Complementary Metal-Oxide-Silicon Integrated Circuits (CMOS ICs). Active/passive circuit techniques can be alternatively used to reduce the threshold voltage of the device. Active technique requires an external power source or secondary battery and is generally used in active sensors or active RFID. This enables more sophisticated applications at the price of increased cost and maintenance. Passive techniques do not require an additional source of energy but may require additional circuit where an auxiliary rectification chain is used to generate the compensating threshold voltage for the main RF-DC power conversion circuit. The auxiliary chain requires additional power and occupies additional area. An internal threshold voltage cancellation circuit was introduced in Kocer et al. where the compensating voltage was generated passively and stored in a capacitor that is applied at the gate-source terminal of the MOS transistor. This technique requires a large resistance and high capacitance value which leads to a large area on the chip. A self-biasing technique consisting of an off-chip high impedance resistive network was used by Li et al. to provide DC biasing voltages. Another technique can be using floating gate transistors storing a pre-charged voltage at the gate lowering its threshold voltage. The threshold voltage can be varied by changing the body-source potential of the transistor. This technique requires an additional pre-charge phase making it unsuitable for fully battery-less applications. The RF-DC power conversion circuit consisting of NMOS transistors with grounded body terminal leads to an increase in the threshold with the number of stages due to the body effect. This degrades the efficiency of the power conversion circuit. The body terminal of the transistors can be dynamically controlled using additional circuit or floating well devices. The floating well technique generates undesirable substrate current. Also, a triple-well source-body connected device has high parasitic capacitance leading to reduced efficiency. A cross-coupled differential scheme was used in Sciorcioni et al. consisting of triple-well NMOS and standard PMOS transistors to reduce the threshold voltage. A self-compensation scheme based on the Dickson topology was introduced in Curty et al. consisting of triple-well NMOS transistors to provide individual body biasing. The compensating voltage was provided by connecting the gate terminal to later stages. The design of the power converter in the works was focused on only reducing the threshold voltage neglecting the key role played by the reverse leakage current in introducing power losses. The reduction in threshold voltage increases the reverse conduction current causing additional loss.
To address the trade-off between the reduced threshold voltage and increased leakage current, Lee et al. proposed use of high speed comparators to control the reverse leakage current. The use of comparator increases the power consumption and limits the usefulness of this technique to low-frequency applications. Dynamic CMOS Dickson pump has been designed in previous works to eliminate the Vth drop while reducing the reverse leakage current. These circuits are designed for digital application with no emphasis on low-power operation. Differential-drive (4T-cell) architecture with the cross coupled bridge configuration and its variant has been used in previous works to reduce the threshold voltage as well as lower the leakage current. Differential circuit requires a PCB balun for the single-ended to differential conversion or differential antenna. Also, the differential circuit requires triple-well NMOS transistors and larger number of rectifying devices for the same number of stages compared with single-ended one.